The field of the disclosure relates generally to high voltage direct current (HVDC) transmission systems and, more particularly, to electric power conversion systems and their methods of operation.
At least some known electric power generation facilities are physically positioned in a remote geographic region or in an area where physical access is difficult. One example includes power generation facilities geographically located in rugged and/or remote terrain, for example, mountainous hillsides, extended distances from the customers, and off-shore, e.g., off-shore wind turbine installations. More specifically, these wind turbines may be physically nested together in a common geographic region to form a wind turbine farm and are electrically coupled to a common alternating current (AC) collector system. Many of these known wind turbines and wind turbine farms are coupled to AC transmission systems through a power converter and a power transformer. Many of the known power converters are a power conversion assembly, or system, electrically coupled to the AC collector system through the power transformer. Such known power conversion assemblies include a rectifier portion that converts the AC generated by the power generation facilities to direct current (DC) and an inverter portion that converts the DC to AC of a predetermined frequency and voltage amplitude.
Some of these known AC collector systems are further coupled to a high voltage DC (HVDC) transmission system through another power transformer and a portion of a separated power conversion assembly, or system. In such configurations, the rectifier portion of the separated power conversion assembly is positioned in close proximity to the associated AC collector systems and the inverter portion of the separated full power conversion assembly is positioned in a remote facility, such as a land-based facility. Such rectifier and inverter portions are typically electrically connected via submerged HVDC electric power cables that at least partially define the HVDC transmission system. Also, at least some known HVDC transmission systems are coupled to DC loads that do not require an inverter portion of AC conversion. Such known AC-to-DC-to-AC power conversion/AC collection/AC-to-DC conversion/HVDC transmission configurations are relatively simply to implement. However, the large number of AC-to-DC-to-AC power convertors significantly increases the costs of construction and maintenance of such configurations.
Similarly, in a second known configuration, a plurality of wind turbines and/or wind turbine farms are coupled to a common DC collection system through a plurality of electric power converters that include an AC-to-DC converter for rectification and a DC-to-DC converter directly coupled to the AC-to-DC converter for voltage boosting and regulation. The common DC collection system is coupled to a HVDC transmission system through another a DC-to-DC converter for further boosting of the DC voltage for long range transmission. Again, the large number of DC-to-DC power convertors significantly increases the costs of construction and maintenance of such configurations.
A third known configuration includes a plurality of wind turbines and/or wind turbine farms directly coupled to a HVDC transmission system through an associated AC-to-DC converter for rectification. The plurality of AC-to-DC converters are distributed into a plurality of strings where the converters in each string are coupled in series, and the strings are coupled to the HVDC transmission system in parallel. Such configurations facilitate high transmission efficiency without the added costs of booster converters. However, the voltages across each of the individual strings must be maintained within a narrow tolerance band to mitigate voltage mismatches and current imbalances between the strings. Maintaining such narrow tolerance bands is exceptionally difficult with the variations in voltage generation typically associated with renewable energy resources, such as wind turbines.